1. Field of the Invention
The invention relates to a method for determining a dopant concentration on a surface and/or in layer region lying close to the surface of a semiconductor sample using an atomic force microscope.
2. Description of the Prior Art
One possibility for doping a semiconductor is the ion implantation method, in which atoms and molecules are ionized, accelerated in an electric field and shot into a solid. The depth of penetration of the ions into the solid depends on their energy, which is typically between several keV and several MeV, and their mass, as well as the mass of the atoms of the solid. Thus, the average range of 10 keV phosphorus ions in silicon is approximately 14 nm and of 1 MeV boron ions in silicon 1.8 μm. By ion bombardment of a solid, it is possible to vary many properties such as electrical and optical properties of a solid, variation in volume, mechanical stresses in the solid surface or the near-surface layer of the solid, as can be deduced from the book by H. Ryssel, I. Ruge: “Ion implantation”, Teubner, Stuttgart, 1978. After doping has been carried out, it is therefore of great interest to determine the dopant concentration in order to obtain better knowledge about the dependence between the dopant distribution inside a solid and the solid properties changed by the doping.
There are therefore a number of techniques for determining the dopant concentration in a solid consisting mainly of semiconductor material, in which atomic force microscopy is used, see also G, Binnig C. F. Quate and C. Gerber, Atomic Force Microscopy, Phys. Rev. Lett. 56, 930-933 (1986), where a small leaf spring having a length of about 100 μm to 500 μm with a tip is scanned by means of piezoelectric adjusting elements over a surface region of a solid sample to be studied, wherein a position sensor measures the deflection of the leaf spring in such a manner that a laser beam is focussed onto the back of the leaf spring, reflected there and deflected onto a photodiode. A bending of the leaf spring effects a variation in the angle of reflection of the laser beam and associated with this, a variation in the photovoltage which can be tapped at the photodiode, by which means the topography of the surface can be imaged, whereby the sensor or the sample is tracked during scanning perpendicular to the sample surface, that is in the z-direction by means of a control loop in such a manner that the deflection of the leaf spring remains constant. The z-voltage is coded as a color value and represented as a topographic image by means of a computer.
The surface topography of the sample can be varied by the process of doping a semiconductor sample using high acceleration voltages. If roughening of the surface occurs, the doped region can be detected by scanning the topography by means of conventional atomic force microscopy. However, with this technique it is difficult to make any prediction of the ion concentration in the space charge zone which is formed after doping.
In the article by P. de Wolf, M Geva, T. Hantschel, W. Vanderworst and R. P. Bylsma, Two-dimensional Carrier Profiling of InP Structures Using Scanning Spreading Resistance Microscopy, Appl. Phys. Lett. 73, 2155-2157 (1998), force microscopy is used to detect charge distributions in semiconductor surfaces in which the tip of a conductive leaf spring is scanned over the surface of a semiconductor element. By applying a static force, the tip is pressed into the semiconductor element to be studied. The contact radius is given by the Hertz contact mechanics and is typically 30 nm. A DC voltage is applied to the sensor bar. The resultant current through the sample is measured on the underside of the sample as a function of the tip position with the aid of a logarithmic amplifier. The overall measured resistance is made up of the sum of the contact resistance and the volume resistance of the sample. By scanning over the surface, an image of the conductivity distribution and ultimately of the doping of wafer or the semiconductor structure can thus be obtained. The three-dimensional resolution of the method is determined by the contact radius of the sensor tip. This technique is also used to characterize pn-junctions and opto-electronic structures and is called “scanning spreading resistance microscopy” (SSRM).
Another technique for detecting doping is so-called “scanning capacitance spectroscopy” (SCM). In this technique the doping can be detected on the one hand by means of a metal-semiconductor contact, that is a Schottky barrier, between the tip and the component, or the tip and/or the component to be studied can be provided with an insulating layer so that the electrical contact resistance remains sufficiently high. The measured quantity in this case is the local electrical capacitance between tip and surface. Charge distributions cause a variation in the capacitance and therefore a contrast for a pictorial representation. The capacitance is determined with the aid of known electrical measuring techniques, see for example J. R. Matey and J. Blanc, Scanning Capacitance Microscopy, J. Appl. Phys, 57, 1437-1444 (1985).
A contact-free method of investigation between leaf spring tip and sample body surface is described by Loppacher et al. by means of so-called “Kelvin Probe Force Microscopy” (KPFM) under vacuum conditions, C Loppacher, U. Zerweck, S. Teich, E. Beyreuther, T. Otto, S. Grafström and L. M. Eng, FM Demodulated Kelin Probe Force Microscopy for Surface Photovoltage Tracking, Nanotechnology 16, pages 1-6 (2005). The local surface potential is detected merely by detecting electrostatic forces between the leaf spring tip and the sample surface, wherein the leaf spring tip and the same surface are not in mechanical contact. The leaf spring is made to oscillate resonantly by applying an electrical alternating voltage between lead spring tip and the sample to be investigated. In addition, a DC voltage is applied between sample and tip in such a manner that the electrostatic forces are compensated to zero. The applied DC voltage is recorded, whereby the surface potential of the sample is obtained, which is dependent on the charge distribution. In addition, the surface can be irradiated with light having a photon energy greater than the band gap of the semiconducting solid or the dopant, whereby electron-hole pairs are formed inside the space charge layer which move towards near-surface regions as a result of different work functions and consequently lead to a variation in the surface potential that can be measured by means of the electrostatic forces. Thus, this technique can be used to obtain information about surface states and charge mobilities such as, for example, diffusion lengths, recombination rates. This method has the advantage that the electrical field between tip and sample is very low, which leaves the electronic band structure of the investigated solid virtually unchanged.
A further method for detecting doped regions applies eddy current techniques in conjunction with magnetic force microscopy (Magnetic Force Microscopy, MFM), see also M. A. Lantz, S. P. Jarvis and H Tokumoto, High Resolution Eddy Current Microscopy, Apply. Phys. Lett. 78, 383-385 (2001). An oscillating leaf spring with a magnetic tip moves over a conductive surface. The oscillation of the leaf spring induces an eddy current field in the sample surface, whose scattered field is again coupled back to the tip. The variation of the oscillation amplitude of the leaf spring gives an indication of the local conductivity of the sample. The resolution of the method is determined by the magnetic scattered field and is a few 100 nm.
Ultrasonic waves can also be detected with the sensor tip of a force microscope and this with the high lateral resolution of a force microscope, as can be deduced from German Patent 43 24 983 B1. In atomic force acoustic microscopy (AFAM) an ultrasonic test head is located under the sample, which emits longitudinal or transverse waves into the sample and thus causes displacements perpendicular to the surface or laterally to the surface. The ultrasonic transducer is connected to a frequency generator which supplies it with a sinusoidal alternating voltage. If the tip of the spring bar is in contact with the sample surface, the oscillations will be transmitted from the sample to the leaf spring. The resonance frequencies of the leaf spring depend on the physical forces acting on the tip. The resonances of the leaf spring in contact with the sample surface are designated as contact resonances of the sample-leaf spring system, see also U. Rabe, K. Janser and W. Arnold, “Vibrations of Free and Surface-Coupled Atomic-Force Microscope Cantilevers: Theory and Experiment”, Rev. Sci. Instr. 67, 3281-3291 (1996). Elastic properties of materials can be determined with the aid of contact resonances. Since dopings influence the elastic properties of semiconductors, these can likewise be detected using the AFAM technique.